The present invention relates to optical fibers used in optically guided airborne bodies such as air-to-ground or ground-to-ground missiles. More specifically, the present invention relates to an apparatus for winding such a fiber on a smooth bobbin.
Optical fiber dispensers are used for communication between a launcher, such as an airborne platform that carries the dispenser and from which the dispenser “pays out” the optical fiber, and a remotely operated object such as a remotely operated missile. The optical fiber is wound on a bobbin, which is normally pre-grooved on its external surface, the grove used to guide the winding of a first bottom layer of the fiber. A detailed description of a prior art dispenser is given for example in U.S. Pat. No. 5,607,532 to LoStracco. For proper, smooth and faultless functioning of the payout, it is necessary that throughout the payout operation the residual pack, i.e. that part of the optical fiber spool that is still on the bobbin, must riot loosen so that each turn of fiber remains in place in its original, slightly tensioned state until payout. The payout may be quite rapid, with speeds higher than 20 m/sec.
An optical fiber is made of at least two different materials, a fused silica inner part which guides the light and an elastomer coating. During winding of the optical fiber on the bobbin an adhesive is applied among the layers of the pack and between the bottom layer and the bobbin. In this way the individual turns of fiber in the pack are fixed so as to ensure that no loosening occurs prior to, and during the payout. The winding action and geometry may themselves cause problems such as optical losses due to micro-bends formed between crossing sections, pressure or tension.
FIG. 1 shows a typical prior art bobbin 10 with a wound fiber 20. For simplicity, only a partial first fiber layer is shown. The bobbin is normally a slightly conical tubular structure that includes a pre-grooved tube 22 having a front end 24 and an aft end 26, with an incline a of about 1-80 along its length axis 28 causing the aft end to have a slightly smaller diameter than the front end. The pre-grooved external surface of the tube includes a helical screw groove 30 for holding and guiding the winding of a first base-layer 32 of fiber 20. The grooves are defined by a pitch P (center-to-center distance of two adjacent grooves) and a depth D. Once the pitch is chosen, fiber 20 must have a slightly smaller (by a few micrometers) diameter to fit in. Thus, typical pre-grooved bobbins can accommodate only fibers of a certain diameter.
Since the use of a pre-grooved tube limits the fiber that can be wound on the bobbing to a fiber with a given diameter tolerance, there is a clear advantage in providing a bobbin that is not pre-grooved. Such a smooth bobbin can then be used for winding fibers with a larger range of diameters, in addition to the saving involved in not needing to pre-groove the bobbin.